Ultrasound systems exist today that utilize a variety of techniques for processing ultrasound signals to generate information of interest. For example, a variety of techniques exist today for performing beamforming upon ultrasound receive signals.
One approach to beamforming performs so called Retrospective Transmit Beamformation (RTBF). This is a transmit focusing technology that achieves dynamic focusing by performing the transmit focusing operation retrospectively.
In Retrospective Transmit Beamforming (RTB), following a transmission event of an ultrasound transmit beam, multiple receive beams are generated. Ultrasound images are composed of a set of lines along each one of which the image system acquires image data until a full frame has been scanned.
Retrospective Transmit Beamforming uses a high parallel line acquisition scheme in which following each transmit event of a transmit beam multiple receive beams are simultaneously acquired in parallel along different lines.
According to this technique, the transmit beam is generated with a width that encompasses multiple receive lines. Generally, this can be achieved by transmitting from a small transmit aperture, for example by transmitting using a lesser number of elements of an array of transducers that the total number of transducer provided in the array. Following transmission, echoes are received which are focused along each line of the lines encompassed by the width of the transmitted beam. Focusing is obtained by delaying and summing the echoes received by the transducer elements of the receive aperture so that for generating the image of each line of the multiple lines encompassed by the width of the transmit beam, only the contribution of coherent signals along each different line location are used.
For scanning the entire image frame and acquiring all the image lines needed for generating the image, further transmit beams are transmitted by shifting the transmit aperture laterally in one direction relatively to the transmit aperture of the previous transmit event.
Lateral shift is carried out in such a way that the two adjacent transmit apertures overlap so that at least some of the receive lines encompassed by the width of a first transmit beams are also encompassed by the width of at least one or more of the following transmit beams which aperture has been progressively laterally shifted in relation to the transmit aperture of the first transmit beam.
As a result, depending on the transmit aperture of the transmission, i.e. of the number of lines encompassed by the width of the transmit beam and on the lateral shift step of the transmit aperture for each following transmit event, lines of image data along each receive line is formed by co-aligned beams along the said receive line which are combined together.
Transmission and reception continues across the image field in this manner until the full image field has been scanned. Each time the maximum number of receive lines for a given line location has been acquired, the receive lines are processed together to produce a line of image data at that location.
Due to the fact that each receive signal contributing to the same image line data at a certain receive line location derives from a transmit beam whose transmit aperture has been shifted with respect to the other transmit beams, the said receive signals contributing to the same line data are not coherent and there is the need of equalizing the phase shift variance that exists from line to line for the multilines with differing transmit-receive beam location combinations, so that signal cancellation will not be caused by phase differences of the combined signals.
In U.S. Pat. No. 8,137,272 a method and an ultrasound apparatus are disclosed which operates according to the above RTB technique.
U.S. Pat. No. 8,137,272 suggests a method for producing an ultrasound image with an extended focal range, comprising the steps of:                transmitting a plurality of transmit beams from an array transducer, each transmit beam being centered at a different position along the array and each transmit beam encompassing a plurality of laterally spaced line positions which are spatially related to laterally spaced line positions of another beam;        receiving echo signals with the array transducer;        concurrently processing the echo signals received in response to one transmit beam to produce a plurality of receive lines of echo signals at the laterally spaced line positions of the beam;        repeating the concurrently processing for additional transmit beams;        equalizing the phase shift variance among receive lines at a common line position resulting from transmit beams of different transmit beam positions;        combining echo signals of receive lines from different transmit beams which are spatially related to a common line position to produce image data; and        producing an image using the image data.        
According to this solution the array transducer elements are connected to a multiline receive beamformer which produces a plurality of receive lines at a plurality of corresponding line positions in response to one transmit beam at each one of a certain number of different beam locations. The multiline receive beamformer operates by using the traditional beamforming technique, namely the so called delay and sum in which the delays are determined by the relative position of the focal point of the transmit beam, the points on the receive line and the transducer position in the array. These delays are determined for all the multiline beamformers along each receiving line according to the same fixed rules which only depend on the time of arrival of the echoes from a reflecting point at a certain transducer element of an array of transducer elements.
So the traditional beamforming delays are based merely on the relative position of the reflecting point and of each of the transducer elements.
The step of equalizing the phase shift variance among receive lines at a common line position resulting from transmit beams of different transmit beam positions is carried out separately from beamforming at a later step. Equalizing further comprises relatively delaying the signals of receive lines along a common line position obtained from different transmit beams prior to combining these receive line signals together in order to receive a beamformed receive data along a certain line position.
According to this method the receive echoes relating to the same line position are firstly selected and summed by the delay and sum process of the multiline beamformer, irrespectively of the possible phase shifts introduced by the shift of the transmit aperture between the beams of the sequence of transmit events.
In a following step the phase shift of the receive signals for the same line location is equalized by a further delay which is determined as a function of the step of lateral shift of the transmit aperture between the transmit events, since the multiple receive signals along the same line position which has to be combined are obtained each one by a transmit beam which has been laterally shifted relatively to the receive line position.
Also a further weighting step of the signals of the receive lines from different transmit beams prior to combining is carried out in a separate step after multiline beamforming.
FIG. 2 shows a system according to the prior art U.S. Pat. No. 8,137,272 and FIG. 3A shows the effect of the equalizing step of the phase shift variance among receive lines at a common line position resulting from transmit beams of different transmit beam positions according to U.S. Pat. No. 8,137,272 and the method and system disclosed therein.
A transducer array comprising a number N of transducer elements of an ultrasound probe is driven by a transmit beamformer in such a way that selected groups of the transducer elements are actuated at respectively delayed times to transmit beams focused at different focal regions along the array. The echoes received by each transducer element of the array in response to each transmit beam are applied to the inputs of a multiline beamformer comprising multiline processors 210a-210k. Each multiline processor 210a-210k processes the each of the k parallel receive lines encompassed by every transmit beam. Each multiline processor comprises a receive beamformer which applies delays 202 and, if desired, apodization weights. The outputs of the multiline processors 210a-210k are coupled to a line memory 212 which stores the received multilines until all of the R multilines needed to form a line of display data have been acquired. The group of multilines used to form a particular line of display data are applied to respective ones of multipliers 216a-216R to produce the display data for the corresponding line location. The echo data from each line may, if desired be weighted by apodization weights 214a-214R. The echoes from each line are weighted by the multipliers 216a-216R and delayed by delay lines 218a-218R. The delays are used to equalize the phase shift variance that exists from line to line for the multilines with differing transmit-receive beam location combinations, so that signal cancellation will not be caused by phase differences of the combined signals. Due to the fixed geometry of the Rx and TX paths and of the lateral shift steps of the transmit aperture in relation to the geometry of the transducer array the delays can be calculated in real time or even calculated in advance and stored in a memory, for example in the former of a table 228.
The delay 218 and a summer 220 effect a refocusing of the signals received from the several receive multilines which are co-aligned in a given direction. The refocusing adjusts for the phase differences resulting from the use of different transmit beam locations for each multiline, preventing undesired phase cancellation in the combined signals.
In FIG. 3A the wavefront of two following transmission events TX1 and TX2 are shown. The transmit aperture of TX2 has been shifted laterally to the right in respect to TX1 by a step corresponding to the dimension of four transducer elements 301 of a transducer array 300. The situation is illustrated in relation to the n-th transducer element as receiving element.
The two wavefronts WF1 and WF2 are in general not planar and a focus P on a receiving line coincident with the center line of the transmission beam the path TX is considered.
According to an embodiment herein the wavefronts are spherical or nearly spherical.
The echoes generated at P has to travel a path RX to reach the n-th transducer element. The Second transmit event generates a wavefront WF2 which reaches the point with a different phase due to the lateral shift of the transmit beam of the transmit event TX2.
As it appears clearly, the receive multiline beamforming delays are defined by the geometry determined by the position of each focus points along each line at each line location relatively to the position of the transducer elements of the transducer array.
The equalization process would require to compensate for the delay of the transmit beam having the wavefront WF2 in reaching the focus points P on the corresponding line, which in FIG. 3A is indicated as RTB delay and is represented by the difference 320 in position of the focus point P of the transmit beam TX1 and the point P1.
In FIG. 4 the effect of the equalization process obtained by the known technique is illustrated schematically.
Considering Bp the beam signal related to focal point p and Sln(t) the signal at the probe channel n, i.e. at the n-th transducer element of the transducer array 300, which is related to a difference “1” in line position between the transmit center line of a second or following transmit beam TX2 and a receive line, then the beam signal can be described by the following equation:
      B    p    =            ∑      l        ⁢                  B        l            ⁡              (                                            2              ⁢                                              p                                                      c                    -                                                                  p                -                                  p                  l                                                                    c                          )            
Where c is the speed of sound and
      t    p    =            2      ⁢                      p                      c  is the time at which the beam signal is focalized at point p, for l=0, which means for coincident center line of the transmit beam TX1 and receive line of the echoes.
The term
                p      -              p        l                  cdefining the delay in reaching the point p of the wave front of the transmit beam according to the l-th shift of the transmit aperture relatively to the transmit beam focused at p in a transmit event in which the receive line is coincident with the transmit beam center line and which is defined above as RTB delay 220.
Further expanding the above equation
      B    p    =                    ∑        l            ⁢                        B          l                ⁡                  (                                                                    p                                            c                        +                                                                            p                  l                                                            c                                )                      =                  ∑        l            ⁢                        ∑          n                ⁢                              s                          l              ,              n                                ⁡                      (                                                                                                                        p                      ^                                        l                                                                    c                            +                                                                                                                                    p                        ^                                            l                                        -                    n                                                                    c                                      )                              
By using the relation
            p      ^        l    =            p      +              p        l              2  it appears clearly that the term
                        p        ^            l            cis the transmission beam path and the term
                                  p          ^                l            -      n            cis the RX path to the nth transducer element of the transducer array.
It has to be noted that in the example of FIGS. 3A and 3B the easiest case has been illustrated in which the receive line RX coincides with the centreline of the first transmit beam of the plurality of laterally shifted transmit beams, so that the distance “l” or the lateral shift step of the centreline of the following or second transmit beam TX2 from the receive line is identical with the lateral shift step between the centre lines of the first and of the second transmit beams TX1, TX2.
As it is shown in FIG. 3B, the effect of the equalisation plus focalization in reception according to the prior art is equivalent, in terms of applied delays in reception, to a traditional focalization by means of the beamforming delays at a point {circumflex over (p)}1 which is half way between point p and point p1. In this case the phase shift introduced by the shifting of the transmit aperture is not exactly compensated. In particular, the focalization delays in reception are computed relatively to the point {circumflex over (p)}1, while the physical propagation delays in reception are proportional to the distance between point P and each transducer n. Therefore, the propagation delays relative to signal backscattered from point P are not exactly compensated by focalization delays applied in reception.